A variety of medical devices have been used or proposed for use for delivering a therapy to and/or monitoring a physiological condition of patients. Some medical devices are entirely or primarily located external to the body of the patient, which others are implantable within the patient. Some medical devices employ stimulation electrodes, sense electrodes, and/or other sensors. Medical devices deliver electrical stimulation to, and/or monitor conditions associated with, the heart, muscle, nerve, brain, stomach, or other organs or tissue. In some examples, electrodes or sensors detect the presence or concentration of proteins or chemicals within the blood or other bodily fluids.
Medical leads are configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, with a proximal portion of the lead coupled to a medical device housing, electrodes or sensors may be located on a distal portion of a lead. Other medical devices include electrodes or sensors on or within the device, or are coupled to sensors wirelessly, and therefore need be coupled to medical leads.
Cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical stimulation to the heart via electrodes carried by one or more leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, a medical device senses intrinsic depolarizations of the heart, and controls delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, an appropriate electrical pacing stimulation signal or signals are delivered to maintain or restore a normal rhythm. In some cases, a medical device delivers rapid pacing pulses to the heart of the patient upon detecting tachycardia. High voltage shocks can be delivered for the purpose of cardioversion of a tachycardia, or for defibrillation of the heart upon detecting fibrillation.
In general, cardiac pacemakers, cardioverters, and/or defibrillators include physiological sense amplifiers coupled to electrodes to detect cardiac electrical signals associated with the depolarization and repolarization the heart, which may be used for a variety of purposes in addition to determining heart rate. For example, cardiac electrical signals may be used for rhythm classification, which may include morphological or other analysis of the signal. As another example, cardiac electrical signals may also be stored for later review by a clinician, e.g., for evaluation or diagnosis. Other implantable or external devices also include physiological sense amplifiers for sensing other physiological signals, such as neurological signals.
The physiological sense amplifiers amplify physiological signals whose amplitudes are typically 20 mV or less, and, in some cases, also filter the sensed physiological signals to increase the signal-to-noise ratio (SNR) prior to processing the signal. However, large voltage signals, such as pacing pulses or shocks for cardioversion or defibrillation, saturate the sense amplifier. Saturation of the amplifier may render the output of the amplifier unusable. For example, when the amplifier is saturated, the information from the sensed signal may not be present in the output of the amplifier. Polarization of the myocardial tissue proximate to electrodes or of the device housing, which can itself act as a stimulation and/or sensing electrode, may also lead to saturation of the sense amplifier.
The time in which it takes the sense amplifier to return from saturation to a normal sensing state may be referred to as the recovery time. The recovery time may last for several seconds. For this reason, the sense amplifier is often blanked during delivery of a pulse or shock to avoid saturation of the sense amplifier. Blanking decouples the sense amplifier inputs from electrodes. Blanking typically extends beyond the duration of the electrical stimulus for a period of time, referred to as a blanking period.